POC: components for ERT

Use this to not announce ecl_well as a dependency across all projects since symbols only end up in a unit test of opm-core.

@bska POC of what we discussed yesterday. this way the find rule can be synced across projects just fine.
I'm not sufficiently well versed in CMake so I'll just ask: What does this (`list(FIND)`) do if a client specifies
```cmake
Find_Package (ERT)
```
i.e., without `COMPONENTS` (with or without `REQUIRED`)?